Method for producing silicon tetrafluoride from uranium oxyfluoride

ABSTRACT

A method for producing silicon tetrafluoride includes combining uranium oxyfluoride and silicon dioxide; heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium; and removing the silicon tetrafluoride.

FIELD OF INVENTION

This invention relates to a method for producing silicon tetrafluoride and more particularly to a method for producing silicon tetrafluoride from uranium oxyfluoride without radioactive carryover.

BACKGROUND OF INVENTION

Silicon tetrafluoride is widely used in the manufacturer of semi-conductor chips, pyrogenic silica, and other industrially important chemicals.

Silicon tetrafluoride can be produced in several ways all of which are based on reacting silica (SiO₂) with either hydrofluoric acid (HF) or fluorosilicic acid (H₂ SiF₆). Thus to produce SiF₄ from SiO₂, the production of either hydrofluoric or fluorosilicic acid intermediate is required.

Current processes to produce SiF₄, in varying degrees of purity, include the reaction of silica with hydrogen fluoride gas according to the reaction:

    SiO.sub.2 (s)+4HF(g)→SiF.sub.4 (g)+2H.sub.2 O

See U.S. Pat. No. 4,382,071.

Purity of the SiF₄ is dependent on the source of the silica and hydrogen fluoride reagents. The reaction is typically carried out at 25°-55° C. in concentrated sulfuric acid (>80% H₂ SO₄) in order to diminish the reverse reaction through capture of the product H₂ O. This process also uses large amounts of HF which raises concerns for corrosion, safety and environmental management.

Production of SiF₄ from fluorosilicic acid can be accomplished according to the reaction:

    SiO.sub.2 (s)+2H.sub.2 SiF.sub.6 (aq)→3SiF.sub.4 (g)+2H.sub.2 O

See U.S. Pat. No. 4,470,959. This reaction is also carried out in concentrated sulfuric acid (>80% H₂ SO₄), but usually at a slightly higher temperature, between 25°-95° C.

It is also possible to produce SiF₄ directly from fluorosilicic acid by thermal decomposition:

    H.sub.2 SiF.sub.6 (aq)→SiF.sub.4 (g)+2HF(g)

However, typical input fluorosilicic acid (20-30% aqueous) comes from fertilizer and phosphoric acid/super phosphate manufacturing waste tails. The fluorosilicic acid is generally low grade containing many impurities such as phosphorous, nitrogen and sulfur, all of which are detrimental to producing high purity SiF₄.

Yet another multi-step process for producing SiF₄ utilizes the reaction of fluorosilicic acid with sodium fluoride and silicon dioxide according to the reaction:

    2H.sub.2 SiF.sub.6 (aq)+6 NaF+SiO.sub.2 →3Na.sub.2 SiF.sub.6 (s)+2H.sub.2 O

followed by thermal treatment of the fluorosilicate salt at 600° C. to release SiF₄ according to the reaction:

    Na.sub.2 SiF.sub.6 →SiF.sub.4 (g)+2NaF

See U.S. Pat. No. 4,615,872.

As with the processes discussed above, this introduces impurities through use of low grade fluorosilicic acid diminishing the purity of the silicon tetrafluoride produced.

However, uranium is a naturally occurring element which is comprised of approximately 0.7%²³⁵ U and 99.3%²³⁸ U. ²³⁵ U is used to produce Nuclear Energy, while ²³⁸ U is not. Because of the low percentage of ²³⁵ U found in naturally occurring uranium, naturally occurring uranium must be enriched in order to obtain sufficient amounts of ²³⁵ U which will support nuclear fission. This enrichment process, aside from producing high concentrations of ²³⁵ U, produces huge amounts of depleted uranium hexafluoride (UF₆ by-product which is a very hazardous compound posing a serious health threat. Since depleted uranium metal is radioactive and expensive to produce, it is used in limited quantities for highly specialized applications. Accordingly, alternative uses are needed in order to avoid having to dispose of the UF₆ at great expense by storing it indefinitely.

One solution to reducing the large stores of UF₆ is to reduce UF₆ to UO₂ F₂ and convert the UO₂ F₂ into SiF₄, and an oxide of uranium, e.g. UO₂,UO₃ or U₃ O₈.

One use for uranium oxide is to add it to the concrete which is used to build bunkers in which radioactive waste is stored in order to provide high density shielding for the stored high level radioactive waste. Uranium oxide possesses outstanding radioactive shielding properties, and when added to concrete in the form of aggregate makes a very effective, low cost shielding material.

Thus, the use of UO₂ F₂ as the fluorine source for production of silicon tetrafluoride eliminates the problems of impurities introduced by the use of fluorosilicic acid. UO₂ F₂ is a relatively inert solid that is easy to handle with proper precaution for containment of low level radioactive materials. UO₂ F₂ is also produced in very high purity, being derived from the highly refined UF₆. Thus, the use of UO₂ F₂ has both technical and economic advantages in the production of high purity SiF₄.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an improved method for producing silicon tetrafluoride.

It is a further object of the present invention to provide such a method which produces silicon tetrafluoride from uranium oxyfluoride.

It is a further object of the present invention to provide such a method which produces silicon tetrafluoride from uranium oxyfluoride without radioactive carryover.

It is a further object of the present invention to provide such a method which produces silicon tetrafluoride which is essentially pure.

It is a further object of the present invention to provide such a method which completely reacts the uranium oxyfluoride into uranium oxide.

It is a further object of the present invention to provide such a method which is cost effective to implement.

The invention results from the realization that a truly efficient and cost effective method for producing silicon tetrafluoride can be achieved by combining uranium oxyfluoride and silicon dioxide as an oxidizing agent, silicon dioxide having a lower thermodynamic stability than any oxide of uranium produced, in stoichiometric proportion so that when the combination is heated to a temperature below the melting point of the uranium oxyfluoride, the uranium oxyfluoride completely reacts with the silicon dioxide to produce silicon tetrafluoride which is free from radioactive carryover and is essentially pure.

The invention features a method for producing silicon tetrafluoride including combining uranium oxyfluoride and silicon dioxide, heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium and removing the silicon tetrafluoride.

In a preferred embodiment combining may include combining the uranium oxyfluoride and the silicon dioxide in stoichiometric proportion. An oxidizing co-reagent may be introduced to the reaction. Removing may include scrubbing the silicon tetrafluoride. Scrubbing may include reacting the silicon tetrafluoride with sodium fluoride. Scrubbing may include reacting the silicon tetrafluoride with an alkaline metal fluoride the alkaline metal fluoride may include potassium fluoride, lithium fluoride or barium fluoride. Heating may include heating the combination in the temperature range of 400° C. to 1000° C. The oxide of uranium oxide produced may include U₃ O₈. The oxide of uranium may include UO₂. The uranium oxyfluoride may include powdered uranium oxyfluoride. The silicon dioxide may include fumed silica. The silicon dioxide may include a mixture of quartz, cristobalite, tridymite, lechatelierite, stishovite, chalcedony, or coesite. The silicon dioxide may include silica glass.

The invention also features silicon tetrafluoride produced by combining uranium oxyfluoride and silicon dioxide, heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium, and removing the silicon tetrafluoride.

The invention also features silicon tetrafluoride produced by combining uranium oxyfluoride and silicon dioxide, heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce a non-radioactive fluorine compound and an oxide of uranium, and removing the uranium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 is a block diagram of the method for producing silicon tetrafluoride according to the present invention;

FIG. 2 is a flow chart of the method for producing silicon tetrafluoride according to the present invention;

FIG. 3 is a schematic diagram of the apparatus used to produce the silicon tetrafluoride according to the present invention;

FIG. 4A is a representation of an x-ray diffraction analysis performed on sodium hexafluorosilicate obtained from the trap residue produced by the apparatus of FIG. 3 according to the method of the present invention;

FIG. 4B is a representation of a standard x-ray diffraction pattern for Na₂ SiF₆ to verify the presence of silicon tetrafluoride in the residue of FIG. 4A;

FIG. 5A is a representation of an x-ray diffraction analysis, similar to FIG. 4, performed on the reaction residue produced by the apparatus of FIG. 3;

FIG. 5B is a representation of a standard x-ray diffraction pattern for U₃ O₈ to verify the presence of uranium oxide in the residue of FIG. 5A;

FIG. 6 is a schematic diagram, similar to FIG. 3, of the apparatus used to produce SiF₄ with the addition of oxygen (as dry air) to the reaction;

FIG. 7A is a representation of an x-ray diffraction analysis, similar to FIG. 4A, performed on the trap residue produced by the apparatus of FIG. 6 according to the method of the present invention;

FIG. 7B is a representation of a standard diffraction pattern for Na₂ SiF₆ to verify the presence of SiF₄ in the residue of FIG. 7A;

FIG. 8A is a representation of an x-ray diffraction analysis, similar to FIG. 5A, performed on the reaction residue produced by the apparatus of FIG. 6; and

FIG. 8B is a representation of a standard x-ray diffraction for U₃ O₈ to verify the presence of uranium oxide in the residue of FIG. 8A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Uranium hexafluoride (UF₆) is reacted with H₂ O to produce uranium oxyfluoride (UO₂ F₂) 10, FIG. 1. The UF₆ reacts with the H₂ O, not necessarily steam, when placed in close proximity to one another such that UO₂ F₂ precipitates from the air directly as the UF₆ reacts with the water vapor. The UO₂ F₂ 10 is combined with silicon dioxide (SiO₂) 12 as an oxidizing agent. SiO₂ is common silica sand and is very abundant and relatively inexpensive. An oxidizing co-reagent 13 may optionally be added to scavenge free oxygen. The combination is heated 14 below the melting point of UO₂ F₂ to produce silicon tetrafluoride (SiF₄) and an oxide of uranium 16 without uranium carryover into the SiF₄. The non-radioactive SiF₄ is then removed 18 leaving only the completely reacted uranium oxide 20. The SiF₄ is then scrubbed and collected 22.

The UO₂ F₂, or uranyl fluoride, which may be granular, is combined in stoichiometric proportion to the SiO₂ oxidizing agent, block 24, FIG. 2. This ensures that the UO₂ F₂ is completely reacted. The SiO₂ may, however, be mixed in slight excess of its stoichiometric proportion. The SiO₂ has a lower thermodynamic stability than any oxide of uranium, such as UO₂, UO₃ or U₃ O₈, which may be produced. Thus, when the combination is heated to sufficiently react the SiO₂ with the UO₂ F₂, block 26, the SiO₂ readily gives up its oxygen and an oxide of uranium and SiF₄ are produced, block 28. Depending on the oxide of uranium desired, an oxidizing co-reagent such as C, CO, or even silicon metal (Si) may be added to the reaction, block 30, shown in phantom. By using Si as the co-reagent the Si reacts with the oxygen produced to make additional SiO₂ which is reacted with the UO₂ F₂.

The combination is heated, block 26, only to a temperature below the melting point of the UO₂ F₂, typically within a temperature range of 400° C. to 1000° C. Heating the combination below the melting point of UO₂ F₂ prevents vaporization of the uranium, ensuring that the uranium does not enter the gaseous phase and thus contaminate the SiF₄ with radioactive uranium, and SiF₄ produced remains essentially pure because of the purity of the products combined.

Heating the stoichiometric combination below the melting point of UO₂ F₂ further ensures that the UO₂ F₂ completely reacts with the SiO₂ oxidizing agent, because no unreacted uranium is lost to the SiF₄. The gaseous SiF₄ and solid oxide of uranium are easily separated so that the nonradioactive SiF₄ may be removed, block 32.

Once the SiF₄ has been removed, block 32, for example by scrubbing the SiF₄ using an absorbent bed or combining the SiF₄ with a reactive agent to separate out the SiF₄, the SiF₄ can be collected, block 34. The SiF₄ may be recovered by heat decomposition of the post scrubbing residue, well known in the art. Scrubbing is not a necessary limitation of the present invention as the SiF₄ may be directly collected. Scrubbing is done primarily to verify the presence and purity of the SiF₄.

Thus, by heating UO₂ F₂ below its melting point ensures not only that the SiF₄ is not contaminated by unreacted uranium, but that all the UO₂ F₂ is reacted due to the stoichiometric proportions in which the combination was mixed, producing a valuable oxide of uranium. The SiO₂, has a lower thermodynamic stability than any oxide of uranium produced, as determined by the enthalpy of formulation, as shown in table 1.

                  TABLE 1                                                          ______________________________________                                                   ENTHALPY OF FORMATION                                                  OXIDE     (-ΔH.sub.f °298) KCAL/MOLE                            ______________________________________                                         U.sub.3 O.sub.8                                                                          854.1                                                                  UO.sub.2                           259.0                                       SiO.sub.2                          217.0                                       UO.sub.3                           294.0                                     ______________________________________                                    

Because SiO₂ has a lower enthalpy of formation than either U₃ O₈, UO₃ or UO₂, it readily gives up its oxygen which is readily accepted by the uranium, which has an affinity for oxygen, to produce UO₂, UO₃ or U₃ O₈, according to the reactions:

    6UO.sub.2 F.sub.2 +3SiO.sub.2 →2U.sub.3 O.sub.8 +3SiF.sub.4 +O.sub.2(1)

    2UO.sub.2 F.sub.2 +(s)+SiO.sub.2 (s)+C(g)→2UO.sub.2 (s)+SiF.sub.4 (g)+CO.sub.2 (g)                                          (2)

    2UO.sub.2 F.sub.2 +SiO.sub.2 +2CO→2UO.sub.2 +SiF.sub.4 +2CO.sub.2(3)

    6UO.sub.2 F.sub.2 +2SiO.sub.2 +Si→2UO.sub.3 O.sub.8 +3SiF.sub.4(4)

    2UO.sub.2 F.sub.2 +2SiO.sub.2 →2UO.sub.3 +SiF.sub.4 (5)

Heating the combination below the melting point of UO₂ F₂ ensures no radioactive carryover into the fluorine compound, as well as no contamination of the fluorine compound.

EXAMPLE 1

An amount of 20.1572 grams of UO₂ F₂ was combined with 2.1461 grams of fumed silicon dioxide (Alfa AESAR SiO₂ --99.8%, -325 mesh, 400 m² /gm). The combination was vigorously shaken for ten minutes to thoroughly mix the combination. The combination 38, FIG. 3, was transferred to a high density, non-porous, alumina (99.8%) ceramic crucible 40. The crucible was then placed at the bottom of larger vessel 42, made of Haynes® HR 160 Alloy material and sealed with corrosion resistant metal cap 44 and Teflon® gasket seal 46 to provide a gas tight enclosure. Cap 44 has inlet ports 48 and outlet port 50 for allowing the gas to flow through vessel 42.

Sealed vessel 42 was placed in a crucible furnace, not shown. Two of inlet ports 48 were capped and the side entry port 48' was used to introduce dry air to the combination at 200-250 cc/min. This flow was maintained throughout the reaction. Vessel 42 was heated to 700° C. in 10°/min. increments and maintained for three hours thereafter. Vessel 42 was cooled to ambient temperature in 20°/min. increments and crucible 40 removed from vessel 42. Post reaction residue 54, shown in phantom, was dark gray and flowed easily from crucible 40. Conversion of UO₂ F₂ to U₃ O₈ was essentially complete, nearly 100% yield as determined by x-ray diffraction analysis of the post reaction residue.

Verification of the SiF₄ may be done in several ways to include collection of the gaseous SiF₄ and performing gas analysis, passing the SiF₄ over an absorbent bed and analyzing the bed material, or chemically reacting the SiF₄ in a trap and analyzing the product using x-ray diffraction.

The reaction occurring in the trap in this particular example is given by the reaction:

    2NaF(s)+SiF.sub.4 (g)→Na.sub.2 SiF.sub.6 (s)        (6)

Similar reactions can also be produced by using other alkaline metal fluorides such as potassium fluoride (KF), lithium fluoride (LiF) and barium fluoride (BaF₂) in the trap.

Verification of SiF₄ was done indirectly using x-ray powder diffraction analysis of the sodium hexafluorosilicate (Na₂ SiF₆) produced in trap 52. X-ray diffraction pattern 62, FIG. 4A, of the trap residue was compared to theoretical reference pattern 64, FIG. 4B, for Na₂ SiF₆. The correlation of peaks 101-116 of trap residue pattern 62 and peaks 101'-116' of reference pattern 64 indicates the presence of Na₂ SiF₆ in the residue. Moreover, the absence of non-correlating peaks confirms that no uranium contaminates the SiF₄.

Accordingly, verification of Na₂ SiF₆ indirectly verifies the presence of SiF₄ based on equation (6) above and confirms that there is no uranium carryover.

Verification of complete reaction of the products to produce oxides of uranium was achieved using x-ray powder diffraction analysis on post reaction residue 54. X-ray diffraction pattern 66, FIG. 5A, of the product residue was compared to standard x-ray diffraction reference pattern 68, FIG. 5B, for U₃ O₈. The correlation of peaks 201-216 of reaction residue pattern 66 with peaks 201'-216' of U₃ O₈ reference pattern 68 indicates the presence of U₃ O₈ in the post reaction residue. Moreover, the absence of non-correlating peaks indicates the absense of unreacted uranium.

EXAMPLE 2

An amount of 20.0115 grams of UO₂ F₂ was combined with 2.1070 grams of SiO₂ The SiO₂ used was Celite® brand pool filter sand consisting of a mixture of silica quartz (CAS#91053-39-3) and cristobalite (CAS#68855-54-9). The SiO₂ may, however, include other naturally occurring mineral forms of SiO₂ including quartz, cristobalite, tridymite, lechatelierite, stishovite, chalcedony, coesite, silica glass or mixtures thereof. The combination was vigorously shaken for ten minutes to thoroughly mix the combination. The combination 36', FIG. 6, was transferred to high density, non-porous, alumina (99.8%) ceramic crucible 40'. The crucible was then placed at the bottom of larger vessel 42', made of Haynes® HR160 Alloy material and sealed with a metal cap 44' and Teflon® gasket seal 46'.

Sealed vessel 42' was placed in a crucible furnace, not shown. For this particular example, one of inlet ports 48' was capped and one was connected to a supply of high purity, dry air including O₂. Trap vessel 52', containing sodium fluoride (NaF) for scrubbing the SiF₄ reaction effluent, was connected to outlet port 50'.

Prior to heating, the flow of air was initiated at 50-100 cc/min and maintained throughout the experiment. Vessel 42' was heated to 700° C. in 10°/min. increments and maintained for four and one half (4.5) hours thereafter. Vessel 42' was cooled to ambient temperature in 20°/min. increments and crucible 40' removed from vessel 42'. Post reaction residue 54', shown in phantom, was brown in color and flowed easily from crucible 40'. Conversion of UO₂ F₂ to U₃ O₈ and was again essentially complete, nearly 100% yield as determined by x-ray diffraction analysis of the post reaction residue.

Verification of SiF₄ was done indirectly using x-ray diffraction on the sodium hexafluorosilicate (Na₂ SiF₆) produced in trap 52'. X-ray diffraction pattern 70, FIG. 7A, of the trap residue was compared to reference pattern 72, FIG. 7B, for Na₂ SiF₆. The correlation of peaks 301-316 of trap residue pattern 70 and peaks 301'-316' of reference pattern 72 indicates the presence of Na₂ SiF₆ in the residue.

Accordingly, verification of Na₂ SiF₆ indirectly verifies the presence of SiF₄ based on equation (6) above and confirms that there is no uranium carryover.

Verification of complete reaction of the products to produce oxides of uranium was achieved using x-ray powder diffraction analysis on post reaction residue 54'. X-ray diffraction pattern 74, FIG. 8A, of the product residue was compared to standard x-ray diffraction reference pattern 76, FIG. 8B, for U₃ O₈. The correlation of peaks 401-409 of reaction residue pattern 74 with peaks 401'-409' of U₃ O₈ reference pattern 76 indicates the presence of U₃ O₈ in the post reaction residue. Moreover, the absence of non-correlating peaks confirms the absence of unreacted uranium.

Thus, the method according to the present invention not only reacts all of the uranium oxyfluoride, but produces a silicon tetrafluoride compound of very high purity by heating the stoichiometric combination below the melting point of the uranium oxyfluoride. The uranium oxyfluoride completely reacts to produce uranium oxide without radioactive carryover into the silicon tetrafluoride compound by-product producing a commercially valuable product without requiring additional processing or generating other low level radioactive wastes. Moreover, producing uranium oxide from uranium oxyfluoride, rather than directly from uranium hexafluoride, produces less radioactive and diluted HF than if all the fluorine in UF₆ was converted to HF.

Although specific features of this invention are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention.

Other embodiments will occur to those skilled in the art and are within the following claims: 

What is claimed is:
 1. A method for producing silicon tetrafluoride comprising:combining uranium oxyfluoride and silicon dioxide; heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium; and removing the silicon tetrafluoride.
 2. The method of claim 1 in which combining includes combining the uranium oxyfluoride and the silicon dioxide in stoichiometric proportion.
 3. The method of claim 1 in further including introducing an oxidizing co-reagent to the reaction.
 4. The method of claim 1 in which heating includes heating the combination in the temperature range of 400° C. to 1000° C.
 5. The method of claim 1 in which the uranium oxide produced includes U₃ O₈.
 6. The method of claim 1 in which the uranium oxide produced includes UO₂.
 7. The method of claim 1 in which the uranium oxyfluoride includes powdered uranium oxyfluoride.
 8. The method of claim 1 in which the silicon dioxide includes fumed silica.
 9. The method of claim 1 in which the silicon dioxide includes a mixture of quartz and cristobalite.
 10. The method of claim 1 in which the silicon dioxide includes at least one of quartz, cristobalite, tridymite, lechatelierite, stishovite, chalcedony or coesite.
 11. The method of claim 1 in which the silicon dioxide includes silica glass.
 12. A method for producing uranium oxide comprising:combining uranium oxyfluoride and silicon dioxide; heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and said silicon dioxide to produce a non-radioactive fluorine compound and an oxide of uranium; and removing said uranium oxide.
 13. A method for producing an alkali or alkaline earth metal hexafluorosilicate comprising:combining uranium oxyfluoride and silicon dioxide; heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium; removing the silicon tetrafluoride; and scrubbing the removed silicon tetrafluoride with an alkali or alkaline earth metal fluoride to produce an alkali metal or an alkaline earth metal hexafluorosilicate.
 14. The method of claim 13 in which the alkali metal or alkaline earth metal fluoride is selected from the group consisting of sodium fluoride, lithium fluoride, barium fluoride and potassium fluoride. 